Estimates of surface heat flux and an index of the surface control over evaporation are made based on radiometric observations of ground temperature. A variational data assimilation approach is used to include surface energy balance in the estimation procedure as a physical constraint (the adjoint technique). This technique formulates the estimation problem as a minimization of ground temperature forecast misfits against observations. The surface energy balance equation is incorporated as a physical constraint. Applications to the First International Satellite Land Surface Climatology Project Field Experiment (FIFE) are presented. The procedure estimates of the surface control over latent heat flux match those values based on independent latent heat flux measurements. The estimates of surface heat flux, when compared with measurements, have a root-mean-square error of 20 W m-2. The need to discriminate between soil wetness and aerodynamic contributions to the surface control over evaporation is recognized. ¿ 1999 American Geophysical Union